Leslea Hlusko: As we come togeth­er as part of this amaz­ing gath­er­ing of peo­ple from around the world, I ask you to pause for a moment to appre­ci­ate the role that our biol­o­gy plays in all of this, from the inno­va­tion of the air­planes that brought many of us here, to just breed­ing the air. We can only do these things because our biol­o­gy enables us to do them. And that biol­o­gy is the result of mil­lions and mil­lions of years of evolution.

I’m going to take you back to two key evo­lu­tion­ary inno­va­tions that hap­pened ear­ly on in our lin­eage that set us off on a tra­jec­to­ry dis­tinct from that of our clos­est liv­ing rel­a­tives, the African great apes, chim­panzees, and goril­las. But before we trav­el back in time, it’s impor­tant that you know that the world 15 mil­lion years ago was a real­ly dif­fer­ent place. There were a lot of apes around, and very very few monkeys.

Now, I don’t mean in terms of sheer num­bers, because of course there are a lot of peo­ple on the plan­et today. I’m talk­ing about the num­bers of species and gen­era, and their geo­graph­ic ranges.

Today when we look at the pri­mates on the plan­et, we see that there are many species of mon­keys around. They’re all over Africa and across Europe and Asia. But when you look at apes, they have a much more restrict­ed geo­graph­ic range, and as a con­se­quence they find them­selves at sig­nif­i­cant risk of extinc­tion. Climate change is cen­tral to this because 10 mil­lion years ago, the Mediterranean dried up. And as those mon­keys and apes were adapt­ing to that major shift in the cli­mate, they were in stiff com­pe­ti­tion with each other.

Research in my lab has led to new dis­cov­er­ies as to how the den­ti­tion was involved in this. Teeth are an essen­tial tool for study­ing evo­lu­tion because they devel­op and erupt over a sig­nif­i­cant por­tion of your life. And also they’re inor­gan­ic, so they pre­serve real­ly well in the fos­sil record.

I’ve been work­ing with a pedi­greed colony of baboons to study the inher­i­tance of den­tal traits. These analy­ses enable us to char­ac­ter­ize evo­lu­tion in a way that reflects how the genet­ic mech­a­nisms that actu­al­ly under­lie den­tal for­ma­tion, how they have evolved through time.

So, we do this by cap­tur­ing the rel­a­tive sizes of teeth. These rel­a­tive sizes cap­ture the out­put of genet­ic pat­tern­ing mech­a­nisms. So if you look on this graph, you can see that the liv­ing mon­keys occupy—they have a lot of vari­a­tion com­pared to the apes. But when we plot on those apes from five to ten mil­lion years ago, you see they actu­al­ly occu­pied space that is only mon­keys today. Those apes were out­com­pet­ed by the monkeys.

Now I’m going to take you to 4.4 mil­lion years ago, to Ethiopia, where we see the fos­sil evi­dence for how our lin­eage sur­vived that com­pe­ti­tion. At the site in [record­ing skips @3:13] Aramis, we have found evi­dence of an ape that had two real­ly unusu­al char­ac­ter­is­tics, both evinced in the shape of the bones of the skele­ton. The pelvis is bowl-shaped, and there had been selec­tion for mus­cu­la­ture that could sta­bi­lize the pelvis while stand­ing on one leg. So these apes could walk like we do.

In con­trast, chim­panzees? They don’t have that, and they walk like this. [takes a few steps, rock­ing from side to side each foot­fall] You see here, this baboon has real­ly large canines? Most male pri­mates have very large canine teeth that they use to fight with each oth­er over social dom­i­nance. And also to fight over sex­u­al access to females. Ardipithecus was different. 

Instead, Ardipithecus males and females both had small canines. So we read from that that these males and females, they were sig­nif­i­cant­ly bond­ed togeth­er. They had pair bond­ing. You don’t see this in any oth­er ape today.

So now, let’s pull all in this togeth­er. So we had these two char­ac­ter­is­tics of the stand­ing on two legs, walk­ing well on two legs, and being pair bond­ed. How do those two things pro­vide an advan­tage in the com­pe­ti­tion with monkeys? 

Well, mon­keys have short­er gen­er­a­tion times. Which means that baby mon­keys grow up and become adult mon­keys much faster than do apes. And so that extend­ed child­hood of apes. It opens up an impor­tant oppor­tu­ni­ty for sig­nif­i­cant parental invest­ment. And that can lead to more com­plex learn­ing. It can lead to lan­guage. And it can lead to adolescence.

Over time in our lin­eage, the males that could car­ry their own off­spring, who could car­ry food and pro­vi­sion, they pro­vid­ed their chil­dren with a sig­nif­i­cant evo­lu­tion­ary advantage. 

So today, when you see behav­iors like this, know that it’s deeply embed­ded in our evo­lu­tion­ary lin­eage. This is the key evo­lu­tion­ary adap­ta­tion that under­lies what it means to be on the human lin­eage. Thank you.